sfriedberg
Diamond
- Joined
- Oct 14, 2010
- Location
- Oregon, USA
There's another thread active about turning eccentric (1/8") long (8') shafts on a manual lathe. I've got a smaller scale problem where I'd like to ask for advice. I plan to teach some highschoolers some rudiments of precision measurement, and part of that is testing for taper and TIR, with bench centers, micrometer and dial (test) indicator. I plan to make several test bars about 6" long, turned and ground between centers. Material will probably be 4140PH. One cylindrical, one with about a 0.001" taper over the length, and ideally one with a cylindrical profile and about a 0.001" eccentricity relative to the center axis. I'm assuming 1/2-3/4" at each end will be turned down below the finished profile for dog driving or chucking. Drilled centers will remain on both ends of each bar.
I am scratching my head on how to do the eccentric bar. There's no way the "drill two centers" approach will work with that little eccentricity. I am considering several options and would appreciate additional suggestions. Whatever I do needs to carry over from the lathe to the tool&cutter grinder (Cincinnati #2). The shop does not have another cylindrical grinder.
Option 1: Turn/grind a cylinder, face off the ends. Indicate position in a V-block in the horizontal mill and drill new centers. This seems like my best option, but there's a whole lot of indicating going on to get the part correctly centered on the spindle before making the desired offset. So highly skill- and patience-dependent.
Option 2: Turn a cylinder between centers. Using 4-jaw chucks both ends, indicate the desired eccentricity, grind to finish. I don't currently have a 4-jaw that fits the work head on my T&C grinder, much less one for a height matched tail stock. But this is possible.
Option 3: Drill center holes on the blank. Build tiny offset center adapters with female center one side, male center offset on the other side, and some way to clamp them in offset alignment at each end of the blank. Turn/grind the blank on the adapters. With my skills and equipment, this option has the best chance of consistently reproducing a specific small offset (if I want to make multiple pieces), but I don't think this will stand up to cutting forces. Might work for the grind step, though.
Option 4: (variation on option 3). Make two disks with an eccentric bore, put them over the turned-down-between-dead-centers drive ends of the blank, clamp them in offset alignment, and chuck/center on the disk ODs. Could use chamfer the disk edges and use matching female centers at both ends. If the disks aren't as wide as the turned-down area, I can arrange a separate drive dog, so the offset disks don't take the cutting force.
I am scratching my head on how to do the eccentric bar. There's no way the "drill two centers" approach will work with that little eccentricity. I am considering several options and would appreciate additional suggestions. Whatever I do needs to carry over from the lathe to the tool&cutter grinder (Cincinnati #2). The shop does not have another cylindrical grinder.
Option 1: Turn/grind a cylinder, face off the ends. Indicate position in a V-block in the horizontal mill and drill new centers. This seems like my best option, but there's a whole lot of indicating going on to get the part correctly centered on the spindle before making the desired offset. So highly skill- and patience-dependent.
Option 2: Turn a cylinder between centers. Using 4-jaw chucks both ends, indicate the desired eccentricity, grind to finish. I don't currently have a 4-jaw that fits the work head on my T&C grinder, much less one for a height matched tail stock. But this is possible.
Option 3: Drill center holes on the blank. Build tiny offset center adapters with female center one side, male center offset on the other side, and some way to clamp them in offset alignment at each end of the blank. Turn/grind the blank on the adapters. With my skills and equipment, this option has the best chance of consistently reproducing a specific small offset (if I want to make multiple pieces), but I don't think this will stand up to cutting forces. Might work for the grind step, though.
Option 4: (variation on option 3). Make two disks with an eccentric bore, put them over the turned-down-between-dead-centers drive ends of the blank, clamp them in offset alignment, and chuck/center on the disk ODs. Could use chamfer the disk edges and use matching female centers at both ends. If the disks aren't as wide as the turned-down area, I can arrange a separate drive dog, so the offset disks don't take the cutting force.